Tag Archives: photoshop

Photoshop plugins : JPEG-LS, JPEG-XR, JPEG-2000

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Last update: July 19, 2015

JPEG

JPEG (file extension.jpg) is a commonly used method of lossy compression for digital images. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality.

The term JPEG is an acronym for the Joint Photographic Experts Group, which was organized in 1986 and created the first standard in 1992. JPEG uses a lossy form of compression based on the discrete cosine transform (DCT). The method lossy means that some original image information is lost and cannot be restored, possibly affecting image quality.

Progressive JPEG

The interlaced Progressive JPEG format compresses data in multiple passes of progressively higher detail. This is ideal for large images that will be displayed while downloading over a slow connection, allowing a reasonable preview after receiving only a portion of the data.

Standard

Standard Photoshop CS2 JPEG Save as options

JPEG-HDR

JPEG-HDR is an extension to the standard JPEG image file format allowing it to store high dynamic range images. It was created by Greg Ward and Maryann Simmons as a way to store high dynamic range images inside a standard JPEG file.

JPEG-XR

JPEG XR is a still-image compression standard and file format for continuous tone photographic images, based on technology originally developed and patented by Microsoft under the name HD Photo. It supports both lossy and lossless compression and is supported in Internet Explorer 9 and alter versions. Today there are not yet cameras that shoot photos in the JPEG XR (.JXR) format.

Lossless JPEG

An optional lossless mode was defined in the JPEG standard in 1993, using a completely different technique as the lossy JPEG standard. The lossless coding process employs a simple predictive coding model called differential pulse code modulation (DPCM). Lossless JPEG has some popularity in medical imaging (DICOM), and is used in DNG and some digital cameras to compress raw images, but otherwise was never widely adopted.

Today the term lossless JPEG is usually used as umbrella term to refer to all lossless image compression schemes including JPEG-LS and JPG 2000.

JPEG-LS

JPEG-LS is a lossless/near-lossless compression standard for continuous-tone images with the official designation ISO-14495-1/ITU-T.87. Part 1 of this standard was finalized in 1999. Besides lossless compression, JPEG-LS also provides a lossy mode (“near-lossless”) where the maximum absolute error can be controlled by the encoder. The filename extension is .jls. Compression for JPEG-LS is generally much faster than JPEG 2000 and much better than the original lossless JPEG standard. The JPEG-LS standard is based on the LOCO-I algorithm (LOw COmplexity LOssless COmpression for Images) developed at Hewlett-Packard Laboratories.

JPEG 2000

JPEG 2000 is an image compression standard and coding system defined in 2000 with the intention of superseding the original discrete cosine transform-based JPEG standard with a newly designed, wavelet-based method. The standardized filename extension is .jp2 or .jpx for the extended part-2 specification. JPEG 2000 is not widely supported in web browsers and therefore not generally used on the Internet. JP2 includes mandatory metadata such as information about an image’s color space. It handles alpha transparency and 16-bit color mode.

Photoshop plugins : JPEG-LS, JPEG-XR, JPEG-2000

JPEG

Photoshop CS2 JPEG-LS plugin by HP

HP offers a copyrighted Photoshop plugin for JPEG-LS. Additional informations about JPEG-LS source code are available at the Wayback Archive Webpage of Aleks Jakulin.

A free JPEG 2000 (j2k) plugin for Photoshop is provided by fnord software, a small software boutique in San Francisco, creating graphics software primarily for Macintosh computers. fnord software provides also a SuperPNG plugin for Adobe Photoshop and a WebM plugin for Adobe Premiere.

JPEG 2000 plugin

JPEG 2000 plugin from fnor software in Photoshop CS2

The advanced features are not working as expected in my Photoshop CS2 version.

Microsoft provides a Photoshop plugin for JPEG-XR

plugin

JPEG-XR plugin from Microsoft in Photoshop CS2

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JPEG Chroma Subsampling

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The JPEG compressed file format can produce significant reductions in file size through lossy compression. The compression techniques take advantage of the limitations of the human eye by discarding additional image details that may not be as noticeable to the human observer.

Humans are much more sensitive to changes in luminance (brightness) than  to chrominance (color) differences. JPEG can discard a lot more color information than luminance in the compression process. Chroma subsampling is the process whereby the color information in the image is sampled at a lower resolution than the original. JPEG translates 8-bit RGB data (Red, Green, Blue) into 8-bit YCbCr data (Luminance, Chroma Blue, Chroma Red).

The different levels of YCbCr subsampling are :

  • 4:4:4 – The resolution of chrominance information is preserved at the same rate as the luminance information. (1×1, subsampling disabled)
  • 4:2:2 – Half of the horizontal resolution in the chrominance is dropped, while the full resolution is retained in the vertical direction, with respect to the luminance. (2×1 chroma subsampling)
  • 4:1:1 – Only a quarter of the chrominance information is preserved in the horizontal direction with respect to the luminance information
  • 4:2:0 – With respect to the information in the luminance channel, the chrominance resolution in both the horizontal and vertical directions is cut in half (2×2 chroma subsampling)

JPEG chroma subsampling is not a particularly good mechanism for compressing images used in the medical field where the chrominance may be equally as important as the luminance.

Photoshop uses different chroma subsampling levels depending on the Quality settings:

  • 2×2 Chroma Subsampling – Save Quality 0-6 or Save For Web Quality 0-50
  • No Chroma Subsampling – Save As Quality 7-12  or Save For Web Quality 51-100

Additional informations about JPEG Chroma subsampling are available at the following links :